A water gun (or water pistol, water blaster, or squirt gun) is a type of toy gun designed to shoot jets of water. Similar to water balloons, the primary purpose of the toy is to soak another person in a recreational game such as a water fight.

Historically, water guns were made of metal and used rubber squeeze bulbs to load and propel water through a nozzle like a Pasteur pipette. While the oldest surviving example of a squirt gun dates to J.W. Wolff's June 30, 1896 patent, depictions of children using water-spraying devices date back to at least the 16th century. In Pieter Bruegel the Elder's painting Children's Games (1560), a child appears to be using a squirt toy to spray water, suggesting early forms of water guns. The oldest known reference to a squirt gun is dated thirty-five years prior to Wolff’s patent, with General William T. Sherman's 1861 quote regarding the effort to quell secession: "Why, you might as well attempt to put out the flames of a burning house with a squirt-gun."

For several years in the United States and Canada, import regulations and domestic laws have required squirt guns to be made of clear or tinted transparent plastic to make them harder to mistake for actual firearms.

Akin to water droppers, the oldest known manufactured water guns utilized a simple rubber squeeze bulb into which water could be drawn, then forcibly expelled out the nozzle by squeezing the bulb rapidly. This design has inherent limitations regarding the amount of pressure that one can achieve (fully dependent on the user's hand gripping strength) as well as the need to refill after each shot.

Many early small water guns used the same trigger based pumping mechanism used for spray bottles. In this type of device, the trigger actuates a positive displacement pump shaft. With the aid of two check valves, often using small ball bearings, fluid is drawn into the pump from a reservoir, then forced out the nozzle upon squeezing the trigger. The simplicity of the spraying mechanism allowed these toys to be manufactured cheaply, and allowed the majority of the body to be used as the reservoir. The primary limitation with this design is the volume of water that can be effectively moved per pump. Increasing pump volume would require more user effort to push the fluid out, making larger designs impractical. However, this technology remains widely used today both in spray bottles as well as small water guns that can be found in a wide variety of shapes and colors.

A piston pumper is another version of this system. They are generally bigger than squirt guns and spray bottles. Piston pumpers do not have triggers. Instead, they fire by moving the pump back and forth. Although the piston pumper has greater output than squirt guns, they tend to be less powerful than pressurized water guns.

Another simple method employed is a syringe or piston type. In essence, the water gun is made up primarily (sometimes exclusively) of the pumping mechanism that comprises an outer pump shaft with an inner pump-rod and water-tight seal. This allows water to be drawn into the pump as the pump is extended, then forcibly ejected out as the pump is compressed. Stream performance is dependent on the user's strength. Some models, like the Super Soaker Power Soaker Jr. and Stream Machines draw in and expel water from their nozzles. This design requires a bucket-type filling source. Other models, like the Super Soaker Power Soaker Mighty Cannon and Water Warriors Steady Stream, have check valves and a reservoir for portability.

During the 1980s, the motorized water gun was perhaps at its most prolific. Companies such as Entertech and Larami created water guns modeled after guns popularized in movies such as Rambo. At the heart of these devices was a small motor and crankshaft that converted a rotary motion into a forward-backward pumping motion to drive a small pump akin to those found in the small spray bottle-type squirt pistols. Stream performance was often not improved, but the motor removed the need to pump, which made the toys popular. The greatest fallbacks were that they wore out batteries quickly and that many trigger action blasters could outdo them, making motorized blasters the lowest class. However, their main strength—and consequent reason for dismissal—was their realistic styling. After some of these realistic-shaped water guns caused accidental shootings by police, stricter rules regarding shapes and coloring of water guns were drafted in the United States.^{[citation needed]}